Janus is the son of Queen Zeal from 12.000 BC Dark Ages who got send through a Time Warp to 600 AD.

There he was found by Ozzie and raised to be the leader of the Mystics/Demons known as Magus.
He kills Cyrus and turns Glenn into a frog and fights a war against the human race.
However, all he wanted was to summon Lavos and destroy it to get revenge for his sister Shala.

When Crono beats him in 600 AD,he accidently gets send back to 12.000 BC and acts as the Prophet for his mother Queen Zeal (who doesn't know the true identity of the Prophet/Magus).
After Queen Zeal summons Lavos ,
Magus tries to kill it and gets defeated.

Later (if you choose to let him live) he will join your party.

After destroying Queen Zeal in the Black Omen, he finally destroys Lavos with Chrono.

Later, he goes back to 600 AD and tries to find a way to find his sister Shala.

He uses scythes and Shadow Magic.
His most powerful attack is Dark Matter.